Merge "Add data-driven tasks description and examples"

userdocs/fuel-user-guide/maintain-environment/data-driven.rst

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+
+.. _data-driven:
+
+Run data-driven tasks
+=====================
+
+You can perform lifecycle management tasks based on the history
+of the cluster states. You can introduce expressions that can be
+computed within the context of cluster configuration, so that you can
+control how the task assignment and execution depending on the configuration
+the changes in the configuration.
+
+This feature uses the `YAQL <https://github.com/openstack/yaql>`_ language.
+
+The YAQL language is extendable and can be used to work with any arbitrary
+structured data format such as JSON or YAML.
+
+Through YAQL you get access to the node's ``deployment_info`` during task's
+serialization.
+
+This feature allows the developer to:
+
+* Identify cluster status ``$.cluster.status`` in [``operational``, ``new``, ``partially_deployed``]
+* Identify node number changes ``changed($.nodes.where($.role = ‘compute’))``
+* Identify cluster settings changes ``changed($.configs.nova)``
+* Identify node settings changes ``changed($.nodes.where($.attribute = ‘r1’).network_scheme(‘routes’))``
+* Identify changes of nodes with particular labels ``changed($.nodes.where($.label = ‘testing’)``
+* Identify expected or deployed value of existing parameters ``new($.configs.nova.parameter) = False old($.configs.nova.parameter) = True``
+* Identify actual value of existing settings ``new($.settings.murano.parameter) = True``
+* Use other valid YAQL expressions
+* Use special YAQL helpers: ``changedAny()`` and ``changedAll()`` which can
+  be used for multiple parameters instead of logical expressions; for example,
+  ``changedAny($.nova, $.mysql, $.network_metadata.vips)``
+
+You can use all these expressions as the fields of deployment tasks.
+Use the prefix ``yaql_exp`` to detect the YAQL expressions.
+
+For example:
+
+.. code-block:: ini
+
+   - id: task-id
+     condition: yaql_exp: {some yaql expression}
+     parameters:
+       data: yaql_exp: {some yaql expression}
+
+An example of the ``condition`` field computation using YAQL:
+
+.. code-block:: ini
+
+   changed($.nodes.where($.status = 'ready' and 'controller' in $.roles))
+   and changed($.nodes.where($.status = 'discover' and 'controller' in
+   $.roles and $.pending_addition = true))
+
+
+This example executes only the selected tasks on the controller nodes.
+
+Another example of how it can be used is a task of a MySQL configuration
+change. For example, for the default deployment you can run a secondary
+database nodes deployment in parallel as there is no risk of data corruption
+or service degradation for the newly created cluster. However during
+the cluster operation it is not acceptable to restart all of
+the secondary MySQL instances on configuration change as this will lead
+to quorum loss and service degradation. You can create an expression for
+the task policy to be parallel to the new deployment and one-by-one
+when operating with the existing cluster.
+